March 27, 2023

Our Sun is a yellow dwarf star, a hot ball of incandescent gas at the heart of our solar system. Its gravity “holds it together” affecting everything from the largest planets to the smallest particles of debris. The connection and interactions between the Sun and the Earth drive the seasons, ocean currents, weather, climate and auroras.

The easiest way to understand the size of the Sun is to compare it to something we know: the Earth, our planet, our home. Many numbers will accompany us in this comparison. Continue to read and find out more about the sun.

The origin of the sun and the planets

The formation of the Solar System dates back to 4.5 billion years ago: in this era all its current protagonists were formed from the same cloud of gas and dust rotating in space. Thanks to the force of gravity, the material gathered around a central nucleus which later became the Sun. The small clusters that were forming in the disk began to collide to move away again, or to merge to form larger clusters. Precisely from these clusters, after millions of years, the present planets originated. Comets and asteroids were born from smaller components. Initially some astronomers thought that the planets formed after the Sun. Today, however, it has been discovered that the planets formed almost simultaneously with the Sun.

Furthermore, given that the matter from which the planets were born was hotter near the Sun, in the inner planets – the closest to the star, in fact – low volatile elements condensed, which led to the formation of rocky planets.


The Sun is the only star whose shape can be appreciated simply by sight, due to its mean apparent angular diameter of 32 ‘ 03 ” of arc, which however varies depending on where the Earth is in the course of its orbit : in fact, it reaches the maximum value (32′ 35″) when our planet is at perihelion , while the minimum value (31’ 31″) at aphelion. Similar apparent dimensions allow, subject to the use of particular instruments and adequate protections, to observe the details of the surface of our star in order to reveal and study the phenomena that characterize it.

With the naked eye it is possible to distinguish the solar disk at sunset or in the presence of fog and clouds, when the light intensity is significantly lower. These observations allow, albeit in rare circumstances, to observe particularly extensive sunspots. Then using a modest telescope, equipped with an adequate filter or used in such a way as to project the image of the star on a white screen, it is possible to easily observe sunspots and flares. However, due to the risks to which the retina is subjectof the eye, the observation of the Sun without the right protections is harmful to sight: in fact, the strong radiation can cause the death of part of the cells of the retina, responsible for vision, or the degeneration of some ocular structures, such as the lens.

The combination of the dimensions and the distance from the Earth of the Sun and the Moon is such that the two stars appear in the sky with more or less the same apparent diameter; this situation is at the origin of periodic occultations of the star by our only natural satellite, which are called solar eclipses ; total eclipses, in particular, allow you to view the solar corona and prominences.

First knowledge

Since its origins, man has made many natural phenomena the object of attention and often veneration, including the Sun. The first astronomical knowledge of prehistoric man, who considered the stars as immutable dots “set” in the celestial sphere, consisted essentially in predicting the motions of the Sun, Moon and planets against the background of fixed stars. An example of this “protoastronomy” is given by the orientations of the first megalithic monuments, which took into account the position of the Sun in the various periods of the year: in particular the megaliths of Nabta Playa (in Egypt ) and Stonehenge(in England ) had been built taking into account the position of the star during the summer solstice. Many other monuments of antiquity were built with the apparent motions of the Sun in mind: one example is the Temple of Kukulkan at Chichén Itzá, Mexico, which was designed to cast shadows in the shape of a snake during the equinoxes.

Development of modern scientific knowledge

One of the first “scientific explanations” of the Sun was provided by the Greek philosopher Anaxagoras. He imagined it as a large sphere of flaming metal larger than the Peloponnese and thought it impossible that it could be dragged by the chariot of the god Helios. For having taught this doctrine, considered heretical, he was accused by the authorities of impiety, imprisoned and sentenced to death (but he was later released due to the intervention of Pericles ).

Eratosthenes of Cyrene was probably the first to accurately calculate the distance of the Earth from the Sun, in the 3rd century BC ; According to what has been handed down to us from Eusebio di Caesarea, he walked the distance from our star in ” σταδίων μυριάδας τετρακοσίας καὶ ὀκτωκισμυρίας ” ( Stadìōn Myrìadas Tetrakosìas Kài Oktōkismyrìas ), or 804 million . very similar to the currently accepted one, from which it differs by just 1%.

Another scientist who challenged the beliefs of his time was Nicolaus Copernicus, who in the 16th century revived and developed the heliocentric theory (which considered the Sun to be the center of the Universe), already postulated in the 2nd century BC by the Greek scientist Aristarchus of Samos. It is also thanks to the work of important seventeenth-century scientists, such as Galileo Galilei, Descartes and Newton, that the heliocentric system finally came to prevail over the geocentric one. Galileo was also the pioneer of solar observation, thanks to the telescope ; the scientistPisano discovered sunspots in 1610, and refuted an alleged demonstration by Scheiner that they were objects transiting between the Earth and the Sun rather than present on the solar surface.

Space missions

With the advent of the space age in the early fifties and the beginning of explorations of the solar system, many probes were specially designed to study our star.

The first satellites designed to observe the Sun were NASA ‘s Pioneer 5 , 6, 7, 8 and 9 , launched between 1959 and 1968. The probes orbited the Sun at a distance slightly shorter than that of the Earth’s orbit and they made the first detailed measurements of the wind and the solar magnetic field. Pioneer 9 operated for a long time, transmitting data until 1987.

In the 1970s, the Helios 1 probe and the Skylab space station provided scientists with significant new data on solar wind emission and the corona. Further data were provided by the NASA Solar Maximum Mission spacecraft, launched in 1980, which had the purpose of observing the ultraviolet, gamma and X -rays emanating from solar flares during the period of maximum activity.

The nineties saw the launch of numerous probes, such as the Japanese Yohkoh (1991), designed to observe solar flares at X-ray wavelengths, and the Solar and Heliospheric Observatory (SOHO, 1995), the result of collaboration between ESA and NASA; the latter in particular has ensured since its launch a constant observation of our star in most of the wavelengths of the electromagnetic spectrum, also allowing the discovery of a large number of grazing comets.

However, these probes have made detailed observations only of the equatorial regions of the Sun, since their orbits were located on the plane of the ecliptic. The Ulysses spacecraft was instead designed to study the polar regions, also making measurements of the solar wind and the intensity of the magnetic field. Launched in 1990, the Ulysses was initially directed towards Jupiter in order to exploit the gravitational slingshot effect of the gas giant and move away from the plane of planetary orbits. In 1998, the TRACE probe was launched, aimed at identifying the connections between the magnetic field of the star and the associated plasma structures, thanks also to the aid of high-resolution images of the photosphere and the lower atmosphere of the Sun.

Location within the Galaxy

The Sun orbits at an estimated distance from the center of the Milky Way26 000 ± 1 400 light years (7.62 ± 0.32 kpc ). The star is located in a peripheral region of the Galaxy, more precisely within the Local Bubble, a cavity in the interstellar medium of the Gould Belt, located in the innermost edge of the Orion Arm, a secondary galactic arm placed between the arm of Perseus and the arm of Sagittarius ; the two arms are separated by about 6500 light-years away. Our star is currently in the Local Interstellar Cloud, a thickening of the interstellar medium due to the merging of the Local Bubble with the adjacent Ring I Bubble. Given the relative distance from the galactic center, from other regions of high stellar density and from strong sources of radiation such as pulsars or similar objects, the Sun, and therefore the solar system, is located in what scientists define galactic habitable zone.

The solar system takes 225–250 million years to complete one revolution around the center of the Galaxy ( galactic year ); therefore the Sun would have completed 20–25 orbits since its formation and 1/1 250 of an orbit since the appearance of the human being on the Earth. The orbital speed of our star is about220km/s ; at this speed, the solar system takes about 1,400 years to travel the distance of one light-year, or 8 days to travel one astronomical unit (au). The apparent direction in which our star moves during its revolution around the center of mass of the Galaxy is called the solar apex and points towards the star Vega and the constellation Hercules, with an inclination of about 60° towards the galactic center.

The orbit of the Sun is believed to have an almost circular elliptical shape, taking into account the perturbations caused by the different distribution of masses in the arms of the galactic spiral; moreover, the Sun oscillates above and below the galactic plane on average 2.7 times each orbit, according to a trend comparable to a harmonic motion. Since the stellar density is quite high in and near the galactic plane, such oscillations often coincide with an increase in the rate of meteor impacts on Earth, sometimes responsible for catastrophic mass extinctions. This increase is due to the fact that the other stars exercisetidal forces on Main – Belt or Kuiper Belt asteroids or Oort Cloud comets, which are consequently directed towards the inner solar system.

The Sun is part of a group of more than 100 million known stars of spectral class G2 within the Milky Way and outshines as many as 85% of the stars in the Galaxy, most of which are faint red dwarfs. Among the closest bright stars, placed within a radius of 17 light years, the Sun occupies the fifth position in terms of intrinsic luminosity : its absolute magnitude, in fact, is equal to +4.83.

Vital cycle

The Sun is a population I (or third generation ) star whose formation would have been induced by the explosion, about 5 billion years ago, of one or more supernova( s) in the vicinity of an extensive molecular cloud of the Orion arm. It is established that, about 4.57 billion years ago, the rapid collapse of the cloud, triggered by supernovae, led to the formation of a generation of very young T Tauri stars, including the Sun, which, soon after its formation, assumed an almost circular orbit around the center of the Milky Way, at an average distance of about26,000 to. The calcium and aluminum-rich inclusions left over from star formation then formed a protoplanetary disk around the nascent star. This hypothesis was formulated in light of the high abundance of heavy elements, such as gold and uranium, in our planetary system. Astronomers believe that these elements were synthesized either through a series of endergonic nuclear processes during the supernova explosion (a phenomenon known as supernova nucleosynthesis ), or thanks totransmutations, by means of successive neutron absorptions, by a massive population II (or second generation ) star.

The Sun is currently in the main sequence of the Hertzsprung-Russell diagram, that is in a long phase of stability during which the celestial body generates energy through the fusion, in its nucleus, of hydrogen into helium ; nuclear fusion also causes the star to be in a state of equilibrium, both hydrostatic , i.e. it neither expands (due to the radiation pressure of thermonuclear reactions ) nor contracts (due to the force of gravity, to which it would naturally be subject), both thermal. A G2-class star like the Sun takes, considering the mass, about 10 billion of years to completely deplete the hydrogen in its core.

The Sun is about halfway along its main sequence. At the end of this period of stability, in about 5 billion years, the Sun will enter a phase of strong instability which takes the name of red giant : when the hydrogen of the nucleus will be totally converted into helium, the layers immediately higher will collapse due to the disappearance of the radiation pressure of thermonuclear reactions. The collapse will determine a thermal increase up to the achievement of temperatures such as to trigger the fusion of the hydrogen in the upper layers, which will cause the expansion of the star up to beyond the orbit of Mercury ; the expansion will cause a cooling of the gas (up to3 500 K ), which is why the star will have a typically intense yellow photospheric colour.


The Sun has a well-defined internal structure, which is not, however, directly observable due to the opacity of the internal layers of the star to electromagnetic radiation. A valid tool for determining the solar structure is provided by helioseismology, a discipline which, exactly like seismology, studies the different propagation of seismic waves to reveal the interior of the Earth, analyzes the different propagation of pressure waves ( infrasound ) that pass through the interior of the Sun. Helioseismological analysis is often associated with simulationscomputerized , which allow astrophysicists to determine with good approximation the internal structure of our star.

The radius of the Sun is the distance between its center and the edge of the photosphere, the layer above which the gases are cold or rarefied enough to allow the radiation of a significant amount of light energy; it is therefore the layer best visible to the naked eye.

The internal structure of the Sun, like that of the other stars, appears to be made up of concentric envelopes; each layer has very specific characteristics and physical conditions, which differentiate it from the next.

The layers are, from the center out:

  • The nucleus ;
  • At the radiative zone ;
  • At the tachocline ;
  • The convective zone ;
  • The photosphere , the surface of the Sun;
  • The atmosphere , divided into:
  • Chromosphere ;
  • Transition Zone ;
  • Corona.


The solar core represents 10% of the star by volume, more than 40% by mass. This is where nuclear fusion reactions take place, the main source of solar energy.

Astrophysicists believe that the solar core is close to 0.2 solar radii in size , with a density greater than150 000 kg/m³ (150 times that of water ), a temperature of approx13 600 000 K (for comparison, the surface temperature of the star is 2 350 times lower – 5 777 K –) and a pressure of almost 500 billion bars ; it is the combination of similar values ​​that favors the nuclear fusion of hydrogen into helium. The core is the only region of our star where nuclear fusion currently [82] takes place. These reactions release energy in the form of γ radiation, which, once emitted by the nucleus, is absorbed and re-emitted by the matter of the upper layers, helping to keep the temperature high; as it passes through the layers of the star, the electromagnetic radiation loses energy, assuming increasingly longer wavelengths, passing from the γ band to the X and ultraviolet band, to then spread in space as visible light. Another product of nuclear reactions are neutrinos, particles that rarely interact with matter and therefore freely traverse space.


The photosphere is the layer of the Sun below which the star becomes opaque to visible light; it is therefore the first visible layer, from which the energy coming from the inside is free to propagate in space. It is home to phenomena such as sunspots and flares. It is characterized by a density of 10 23 particles per cubic meter (equivalent to 1% of the density of the Earth’s atmosphere at sea level), while its thickness varies from a few tens up to a few hundreds of kilometers.

The change in opacity with respect to the lower layers (its opacity is in fact slightly lower than that of the Earth’s atmosphere is due to the decrease in the number of hydride ions (H − ), which easily absorb visible light; the light we perceive is instead produced by the recombination between free electrons and hydrogen atoms to generate H − ions.

Since the uppermost layers of the photosphere are colder than the deeper ones, the image of the Sun appears brightest in the center, and gradually becomes more tenuous as one proceeds towards the edge of the perimeter of the visible disk; this phenomenon is called edge darkening, and is caused by a perspective phenomenon.

The photospheric spectrum has characteristics relatively similar to those of the continuous spectrum of a black body heated to the temperature of5 777 K, and appears interspersed with the absorption lines of the tenuous stellar atmosphere. Upon direct observation, the photosphere has a grainy appearance, due to the presence of granulation and supergranulation. During early studies of the optical spectrum of the photosphere, some absorption lines were found that did not correspond to any known element on Earth. In 1868, Norman Lockyer hypothesized that these lines were caused by a new element, which he called helium, after the Greek sun god of the same name; twenty-five years later, helium was isolated on Earth.

The radiative zone

Located outside the nucleus, the radiative zone extends from about 0.2 to 0.7 solar radii; it absorbs the energy produced by the nucleus and transmits it by radiation (hence the name) to the upper layers. Pressure and temperature are still high enough to allow energy transfer to the next layer.

In this band the transfer of the energy produced in the core takes place towards the upper layer, the convective zone; the radiative zone appears devoid of convective motions : in fact, while the matter becomes colder at increasing altitudes, the temperature gradient remains lower than that of the adiabatic rate of fall , which facilitates the transfer of energy by radiation.

Energy is transferred to the outermost layers very slowly: in fact, hydrogen and helium ions emit photons , which travel a short distance before being reabsorbed and re-emitted by other ions.

A recent analysis of data collected by the SOHO mission suggests that the rotational speed of the radiative zone is slightly lower than that of the nucleus.


The layers above the photosphere constitute the solar atmosphere and are visible at all wavelengths of the electromagnetic spectrum, from radio waves to gamma rays passing through visible light. The layers are, in order: the chromosphere, the transition zone, the corona, and the heliosphere ; the latter, which can be considered the slight continuation of the crown, extends beyond the Kuiper belt, up to the heliopausewhere it forms a strong bow shock with the interstellar medium. The chromosphere, transition zone, and corona are much hotter than the solar surface; the reason for this warming is still unknown.

Here is also the coldest layer of the Sun: it is a band called the region of minimum temperature, located approximately 500 km above the photosphere: this area, which has a temperature of4 000 K, is cold enough to allow the existence of some molecules, such as carbon monoxide and water, whose absorption lines are clearly visible in the solar spectrum.

Magnetic field

The turbulent motion of the plasma and of the charged particles of the convective zone generate a powerful magnetic field, characterized by paired poles (north and south) arranged along the entire solar surface. The field reverses its direction every eleven years, at the maximum of the solar cycle. The solar magnetic field is at the origin of various phenomena which collectively take the name of ” solar activity “; among them are the photospheric spots, the flares (or flares) and the variations in the intensity of the solar wind, which scatters matterthrough the solar system.

The differential rotation of the star causes a strong deformation of the magnetic field lines, which appear tangled on themselves; [98] the plasma of solar flares is arranged on them, forming vast rings of incandescent matter, known as coronal rings. The deformations of the field lines give rise to the dynamo and to the eleven-year cycle of solar activity, during which the intensity of the magnetic field undergoes variations.

The solar magnetic flux density is 10 −4 tesla in the vicinity of the star.

The interaction between the solar magnetic field and the plasma of the interplanetary medium creates a diffuse heliospheric current, i.e. a plane that separates regions where the magnetic field converges in different directions.

Chemical composition

The Sun, like every other celestial body in the Universe, is made up of chemical elements. Many scientists have analyzed these elements to understand their abundance, their relationships with the constituent elements of the planets and their distribution within the star.

The star has “inherited” its chemical composition from the interstellar medium from which it originated: hydrogen and helium, which make up the great part, were formed thanks to the Big Bang nucleosynthesis , the heavier elements were synthesized by the nucleosynthesis of the most evolved stars, which, at the end of their evolution, spread them in the surrounding space. The composition of the nucleus is strongly altered by nuclear fusion processes, which have increased the mass percentage of helium (34%) to the detriment of hydrogen (64%). The percentage of heavy elements, conventionally calledmetals, remained practically unchanged. These, present in traces above all in the more superficial layers, are: lithium, beryllium and boron ; neon, the actual quantity of which would be greater than previously estimated through helioseismological observations; the elements of group 8 of the periodic table, to which iron, cobalt and manganese belong. Numerous astrophysicists have also taken into consideration the existence of mass fractionation relationships between the compositionsisotopes of the noble gases, such as neon and xenon, present in the solar and planetary atmospheres.

Since the interior parts of the star are radiative and non-convective, the photosphere, consisting essentially of hydrogen (about 74% by mass, 92% by volume ), helium (about 24-25% by mass, 7% volume) and trace elements, has maintained and maintains a chemical composition essentially unchanged since the formation of the star, [80] so much so that many tend to consider it as an example of the primordial chemical composition of the solar system.

Until 1983 it was widely believed that the star had the same composition as its atmosphere; in that year it was discovered that the very fractionation of the elements in the Sun was at the origin of their distribution within it. This fractionation is determined by various factors, such as gravity , which causes the heavier elements (such as helium, in the absence of other heavier elements) to arrange themselves in the center of mass of the celestial body, while the elements less heavy (therefore hydrogen) diffuse through the outer layers of the Sun; the diffusion of helium within the Sun tends to speed up over time.

In mythology and religion

In many ancient cultures, starting in prehistory, the Sun was conceived as a deity or supernatural phenomenon; the cult paid to it was central to many civilizations, such as the Inca, in South America, and the Aztec, in Mexico.

In Egyptian religion the Sun was the most important deity; the pharaoh himself, considered a deity on earth, was believed to be the son of the Sun. The most ancient solar deities were Wadjet, Sekhmet, Hathor, Nut, Bastet, Bat and Menhit. Hathor (later identified with Isis ) fathered and cared for Horus (later identified with Ra ). The motions of the Sun in the sky represented, according to the conception of the time, a struggle waged by the soul of the pharaoh and Osiris. The assimilation of some local deities (Hnum-Ra, Min-Ra, Amon-Ra) to the solar cult reached its peak in the time of the Fifth Dynasty.

During the Eighteenth Dynasty, Pharaoh Akhenaten attempted to transform Egypt’s traditional polytheistic religion into a pseudo- monotheistic one, known as Atonism. All deities, including Amun, were replaced by the Aten, the solar deity who ruled over Akhenaten’s region. Unlike the other deities, Aten did not possess multiple forms: the only effigy of him was the solar disk. This cult did not survive long after the death of the pharaoh who introduced it and soon the traditional polytheism was reaffirmed by the same priestly caste, which some time before had embraced the atonistic cult.

In Greek mythology the principal solar deity was Helios, son of the Titans Hyperion and Theia. The god is normally represented driving the chariot of the sun, a quadriga pulled by horses that emit fire from their nostrils. The chariot rose every morning from the Ocean and pulled the Sun across the sky, from east to west, where the two palaces of the god were located. In more recent times, Aelius was assimilated to Apollo.

In literature and music

In culture, the Sun is mainly used as a mythological and mystical- religious reference, more than in literature: in fact, unlike the stars, which are mentioned as nocturnal marvels by poets and men of letters, the Sun in literature is mainly used as a reference for the alternation of day and night. However, there are strong references specifically dedicated to this star in literature, painting and even music.

One of the most famous and also the oldest texts in Italian literature that refers to the Sun is in the Cantico di Frate Sole, also known as the Canticle of the Creatures written by St. Francis of Assisi, completed, according to legend, two years before his death, which took place in 1226. The Canticle is a praise to God, a prayer permeated by a positive vision of nature, since the image of the Creator is reflected in creation. With the birth of historiographical science, between the eighteenth and nineteenth centuries and with romantic idealsof the “popular roots of poetry”, the work was taken into consideration by the critical and philological tradition.

Even Dante Alighieri, as a good connoisseur of astronomy, does not fail to mention the Sun in his works, using it as an astronomical reference: in the First Canto of Paradise, for example, he describes the light of the Sun, explaining that since it illuminates the hemisphere in which Purgatory is located, the city of Jerusalem, which is on the opposite side of the Earth, is at that moment immersed in the darkness of the night. Dante thus pauses to observe the splendor of our star, imitating his guide, Beatrice.

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